Combining chapters 17 and 21, Luke records 23 end-time signs that Jesus prophesied will take place before and around the time of his coming. This is Luke's account of the Olivet discourse.
Luke 21 is the second half of Luke's account of the Olivet Discourse, in which Jesus describes end-time signs.

Please also see my commentary on Matthew 24, which is the most detailed account of the Olivet Discourse. From Matthew 24 and other passages, it is helpful to understand that Jesus describes many signs which will occur over the lifespan of an end-time generation that will ultimately witness his second coming. Seven years before the second coming, the Antichrist will confirm an international peace treaty (Daniel 9:27). The signing of this treaty is an important sign to watch out for. Some scholars call this the beginning of the Tribulation. Three and a half years later, the Antichrist will break his treaty, invade Israel and Jerusalem, and set up what is known as the abomination of desolation in the Jerusalem temple. This event marks the beginning of a time of great suffering known as 'The Great Tribulation' (Matthew 24:21, Luke 21:23), or as the 'Time of Jacob's Trouble' (Jeremiah 30:7), or as the 'Days of Vengeance' (Isaiah 61:2, Luke 21:22). The Great Tribulation, is a clearly defined time-period in scripture. It begins with the 'abomination of desolation' (Matthew 24:15-21 and Daniel 12:11) and lasts for three and a half years (Daniel 7:25, 12:7, Revelation 12:14), or 42 months (Revelation 11:2, 13:5) or 1,260 days (Revelation 11:3 and 12:6). Taking a Jewish year as 360 days, and a month as 30 days, all three of these lengths are equivalent. The Great Tribulation is a time of world war, set off my Antichrist's invasion of Israel. It is a time of great distress for the whole world (Luke 21:35), and of wrath against the Jews (Luke 21:23). The second coming marks the end of the Great Tribulation, when Jesus directly intervenes in this war. The war then continues for a relatively short period of time. During this Post-Tribulation period, Jesus fights and defeats the Antichrist and his armies, and delivers Israel.

With such an understanding, it is helpful to try to categorise the end-time signs described in Luke 17 and 21, as follows:

Pre-Tribulation Signs
1) Persecution, hatred and martyrdom of Christians (21:12-18)
2) Fig tree sprouts leaves, symbolic of Israel's restoration (21:29)
3) False Messiahs saying the time is near (21:8)
4) Wars and rebellions (21:9-10)
5) Great earthquakes (21:11)
6) Famines (21:11)
7) Plagues (21:11)
8) Terrifying sights (21:11)
9) Great signs in the sky (21:11)
10) End-time generation characterised by violence (17:26) and immorality (17:28)
11) End-time generation oblivious to coming judgment (17:27-29)

Great-Tribulation Signs
12) Jerusalem surrounded by armies (21:20)
13) Jews flee Israel and Jerusalem (21:21-23)
14) Jews killed or led away as captives among all nations (21:24)
15) Jerusalem trampled by the Gentiles (21:24)
16) Tsunamis (21:25)
17) People fainting from fear (21:26)
18) Heavenly bodies appear shaken (21:26)

Post-Tribulation Signs
19) The world sees Jesus arriving in a cloud with power and great glory (21:27)
20) Second coming as visible as lighting when it finally happens (17:24)
21) Signs in the sun, moon and stars (21:25)
22) Targeted judgments against individuals (17:34-35)
23) Vultures gathering, symbolic of the aftermath of the battle of Armageddon (17:37)

Comparison of Luke 17 and 21 with Matthew 24
Matthew lists most of these signs and more. However, in verse 11 Luke adds 'plagues' which were perhaps of more interest to Luke who was a doctor than they were to Matthew. Luke also adds the word 'great', in relation to earthquakes, famines and plagues, emphasising the unusual magnitude of these three events. The same verse in Luke also mentions 'terrifying sights and great signs from heaven'. These would appear also to be 'Pre-Tribulation signs'. In verse 25 he mentions 'signs in the sun, moon and stars' which correspond to post-tribulation signs associated with the battle of Armageddon in Matthew 24. The 'great signs from heaven' in verse 11 would appear to be distinct Pre-Tribulation signs in the sky, and not the same as those in verse 25. In verses 12-19 Luke describes the persecution and betrayal of Christians, introducing it with 'But before all this'. In other words, he places persecution as the first of the Pre-Tribulation signs. In verse 25, Luke also adds tsunamis, saying "…and on the earth nations will be in distress, anxious over the roaring of the sea and the surging waves". In verse 26, he adds detail about people's reactions to the heavenly bodies being shaken, saying "People will be fainting from fear and from the expectation of what is coming on the world". In verse 20, Luke introduces the Great Tribulation with Jerusalem being surrounded by armies, instead of mentioning the abomination that causes desolation. In verse 24 he describes Jews being killed and taken as captives to all the nations. In verse 35, Luke emphasises that the Great Tribulation (that day) will affect everyone on the face of the earth. In summary, Luke adds the following pre-tribulation signs:

-Plagues (v11)
-Terrifying sights (v11)
-Great signs in the sky (v11)
-Tsunamis (v25)

and he adds the following Great-Tribulation signs:

-Jerusalem surrounded by armies (v20)
-Jews killed or led away as captives among all nations (v24)
-Great Tribulation will affect everyone on the face of the whole earth (v35)
Places: Jerusalem
Symbols: Fig tree, Vultures
Tags: End-time signs, Earthquake, Famines, Plagues, Tsunamis, Wars and rumours of war, Signs in the sky, Restoration of Israel, Fall of Jerusalem, Persecution of Christians, Deception, False messiahs, End-time exile, Sun and moon darkened, Second coming
The Widow’s Offering
21 Jesus looked up and saw the rich putting their gifts into the offering box.
2 He also saw a poor widow put in two small copper coins.
3 He said, “I tell you the truth, this poor widow has put in more than all of them.
4 For they all offered their gifts out of their wealth. But she, out of her poverty, put in everything she had to live on.”

The Signs of the End of the Age
5 Now while some were speaking about the temple, how it was adorned with beautiful stones and offerings, Jesus said,
6 “As for these things that you are gazing at, the days will come when not one stone will be left on another. All will be torn down!”
7 So they asked him, “Teacher, when will these things happen? And what will be the sign that these things are about to take place?”
8 He said, “Watch out that you are not misled. For many will come in my name, saying, ‘I am he,’ and, ‘The time is near.’ Do not follow them!
9 And when you hear of wars and rebellions, do not be afraid. For these things must happen first, but the end will not come at once.”

Persecution of Disciples
10 Then he said to them, “Nation will rise up in arms against nation, and kingdom against kingdom.
11 There will be great earthquakes, and famines and plagues in various places, and there will be terrifying sights and great signs from heaven.
12 But before all this, they will seize you and persecute you, handing you over to the synagogues and prisons. You will be brought before kings and governors because of my name.
13 This will be a time for you to serve as witnesses.
14 Therefore be resolved not to rehearse ahead of time how to make your defense.
15 For I will give you the words along with the wisdom that none of your adversaries will be able to withstand or contradict.
16 You will be betrayed even by parents, brothers, relatives, and friends, and they will have some of you put to death.
17 You will be hated by everyone because of my name.
18 Yet not a hair of your head will perish.
19 By your endurance you will gain your lives.

The Desolation of Jerusalem
20 “But when you see Jerusalem surrounded by armies, then know that its desolation has come near.
21 Then those who are in Judea must flee to the mountains. Those who are inside the city must depart. Those who are out in the country must not enter it,
22 because these are days of vengeance, to fulfill all that is written.
23 Woe to those who are pregnant and to those who are nursing their babies in those days! For there will be great distress on the earth and wrath against this people.
24 They will fall by the edge of the sword and be led away as captives among all nations. Jerusalem will be trampled down by the Gentiles until the times of the Gentiles are fulfilled.

The Arrival of the Son of Man
25 “And there will be signs in the sun and moon and stars, and on the earth nations will be in distress, anxious over the roaring of the sea and the surging waves.
26 People will be fainting from fear and from the expectation of what is coming on the world, for the powers of the heavens will be shaken.
27 Then they will see the Son of Man arriving in a cloud with power and great glory.
28 But when these things begin to happen, stand up and raise your heads, because your redemption is drawing near.”

The Parable of the Fig Tree
29 Then he told them a parable: “Look at the fig tree and all the other trees.
30 When they sprout leaves, you see for yourselves and know that summer is now near.
31 So also you, when you see these things happening, know that the kingdom of God is near.
32 I tell you the truth, this generation will not pass away until all these things take place.
33 Heaven and earth will pass away, but my words will never pass away.

Be Ready!
34 “But be on your guard so that your hearts are not weighed down with dissipation and drunkenness and the worries of this life, and that day close down upon you suddenly like a trap.
35 For it will overtake all who live on the face of the whole earth.
36 But stay alert at all times, praying that you may have strength to escape all these things that must happen, and to stand before the Son of Man.”
37 So every day Jesus was teaching in the temple courts, but at night he went and stayed on the Mount of Olives.
38 And all the people came to him early in the morning to listen to him in the temple courts.